1. Field of the Invention
This invention relates to a method of extruding thermoplastic resin and an apparatus therefor, and more particularly to a method of extruding thermoplastic resin, wherein heat generated by viscous energy dissipation of the resin is low, quantitatively stabilized and high speed extrusion of the resin can be effected at low resin temperature and low resin pressure, and the extruded resin thus obtained is excellent in transparency and gloss, and an apparatus therefor.
2. Description of the Prior Art
To mold the thermoplastic resin into a film, a sheet or a blow molding article, there have heretofore been many cases where the thermoplastic resin is supplied to an extruder having as its principal elements a cylinder and a screw rotating in the cylinder, the resin is continuously heated, melted, kneaded and finally extruded through a die of a predetermined shape.
Screws of various types are known as ones used in the extruder used according to this exclusive method. For example, the screws generally known include: one of the metering type, in which the depth of screw channel rapidly changes in the longitudinal direction thereof; one of the type, in which the depth of the screw channel is progressively decreased and the pitch is formed at a predetermined interval; one of the type, in which the depth of screw channel is constant and the pitch is progressively decreased; and one of the type, in which the screw is provided at the forward end thereof with a torpedo head or a Dulmage head.
All of the constructions of the above-described screw are designed with the main purpose of fully effecting the kneading of the molten resin, whereby the compression ratio is high. Therefore, as for the kneading, the constructions of the screw are effective, however, on the other hand, the following disadvantages will be derived.
In short, the fully effected kneading during the high speed extrusion presents such disadvantages that the resin is subjected to a high shear stress and the resin temperature is raised, whereby the resin is lowered in its viscosity, so that the resin properties are deteriorated, further, the properties of products are deteriorated and the extruding stability is lowered, thereby enabling to stably obtain the satisfactory products. Furthermore, in the case of the high speed extrusion, the driving force required for driving the screw is high, whereby this driving force causes the resin temperature to be raised. In consequence, in order to prevent the resin temperature from being overheated, it becomes necessary to cool the resin from the outside. These problems are highly disadvantageous in that undesired equipment for energy saving and cooling is required.
Particularly, these problems are severe in the case of the thermoplastic resins, showing the Newtonian properties (showing a relatively high melting viscosity under high shear rate) caused by a relatively narrow molecular-weight distribution, and including polypropylene, linear low density polyethylene, polycarbonate, polystyrene and the like.
To solve the above-described problems, there have recently been proposed a method of using a screw being low in compression ratio, particularly, having a compression ratio of 1 or less, and a method of using a two-stage screw.
According to the former method, high shear of the resin can be certainly avoided, however, on the other hand, uniform melting, deaerating and high-speed stable extrusion of the resin cannot be performed, so that stabilized part quality cannot be obtained. As a consequence, it cannot be said that this method is practicable. According to the latter method, an ordinary screw is formed into two-staged ones, which are mainly used in a vent type extruder. However, in this vent type extruder, kneading portions such as a torpedo head or a Dulmage head, where high shear occurs, are provided on the forward end portions of respective stage screws, and this vent type extruder is not substantially different from the ordinary extruders.
As described above, according to any one of these practicable methods of the prior art, anyway, such extruders are used, in each of which high shear occurs at the forward end portion of the screw, and moreover, the resin pressure is raised. As a consequence, according to these extruding methods, the resin pressure and the resin temperature become relatively high, and the resin is extruded from a die with the shear stress caused by the shearing in the extruder being not relaxed, whereby the swell occurs at the die outlet. Deformation due to this die swell and cooling of the molten resin occur simultaneously, whereby the surface conditions of the molten resin are deteriorated, so that such satisfactory extruded stocks being excellent in transparency and surface gloss and low in swell have not been obtainable. As a consequence, when this extruded stock is formed and cooled to provide a film, a sheet, a hollow container or the like, a satisfactory product has not always been obtained.